284 Electronic Unit Injectors & Unit Pumps (EUI’s & EUP’s) Introduction The demands for cleaner engine emissions, improved power, quieter operation and better fuel economy has helped inspire the development of even more efficient fuel injection systems. The unit injection system integrates the injection pump and nozzle into a single module. This means the functions of fuel pressurization; injection timing, atomization and fuel distribution are accomplished using one single component. Above - Unit injection using overhead camshaft and pushrod actuation by camshaft. Description and advantages Unit injectors are installed directly into the cylinder head above each combustion chamber. The injector is driven by the engine camshaft, most often directly through only a rocker lever as in the case of an overhead camshaft. This configuration is one of the most efficient hydraulic and mechanical fuel system arrangements providing the low parasitic losses. Actuating the injectors using an overhead camshaft permits the least amount of injection lag since there are no push-tubes to bend, injector lines to swell or delivery vales to lift. The short distance pressurized fuel travels means an absence of injection lag due to high-pressure fuel wave propagation found in PLN systems. Eliminating injection lag provides for precise injection timing and facilitates the most valuable attribute of these injectors - the highest spray in pressures of any fuel injection system. Presently, the highest injection pressures are only available from unit injectors. The largest injectors currently pressurize to over 30,000-psi. This is about 20% higher than current common rail injection systems. Unit injection systems with 33,000-psi pressures 285 are planned. However, these high spray–in pressures are dependent on engine speed. Plunger velocities must be at their fastest to achieve these pressures. Specialized construction features in the smaller automotive injectors provide good spray-in injection pressures during low speed operation. In older, heavy-duty diesel engine applications, electronic unit injector may be driven through a pushrod type actuating mechanism. Push rods will solid and short to minimize bending and flexing causing injection lag and timing variations. The extremely fine spray produced by unit injectors results in the best combustion chamber atomization and fuel distribution. Conditions like this is permits more effective and leaner upper-cylinder combustion. Consequently, unit injectors can produce more power and torque for a given quantity of fuel without a penalty to fuel consumption and emissions. For example, Volkswagens 2004 100 hp 1.9.litres TDI unit injected engine delivers 10 per cent more power and 13 percent more torque over the electronically controlled radial piston distributor pump system it replaced. Even better is fuel economy increases 9% in city (46 to 50-mpg Imperial 6.2 l/100 km to5.6 l/100 km) and 3-mpg highway (61 to 64-mpg imperia l4.6 l/100 km to 4.4 l/100 km). Fuel droplet sizing of less than 20 microns allows for improved emissions characteristics. This includes less particulate matter (PM) hydrocarbons (HC) and carbon monoxide (C0). Finer atomization promotes tolerance of EGR flow into the combustion mixtures and the use of retarded engine timing for N0x reduction. Advantages of EUI’s Finer atomization produces better fuel economy and lower particulate emissions. 286 Construction of EUI’s Electronic unit injectors are mechanically pressurized using electronic control of governing, timing and metering functions. A unit injector consists of several basic elements. 1. The spring-loaded plunger and barrel assembly to pressurize fuel inside the injector. 2. A poppet valve, known also as a spill control valve or solenoid needle valve that regulates the build-up of pressure inside the injector. 3. An electric solenoid or cartridge valve that actuates movement of the poppet valve or needle valve. Linkage such as a stator or solenoid pin may also directly connect the solenoid to the poppet valve in some injectors. 4. Fuel inlet and return passageways. The fuel inlet is supplied low-pressure fuel by the transfer pump. The return passageway permits fuel which has circulated through the injector to flow back to the supply tank. Since the injector is located in the cylinder head, and pressurization of fuel creates heat, it is important that a steady circulation of fuel passes through the injector removes heat. 5. The nozzle valve at the injector tip is like a conventional nozzle valve opening at 4,500-psi to 5000-psi in most injectors. This provides for improved atomization and crisp beginning and end of injection characteristics. Below- Simplified operation of EUI’s. An electromagnetically controlled spill valve opens and closes to determine the beginning and end of injection. 287 EUI Operating Principles Injection metering and timing Regardless of the type of unit injector, EUI’s share common operating principles. An electrical signal is sent to the unit injector’s solenoid-actuated spill/poppet valve system to deliver the correct quantity fuel at the precise time required to by ECM algorithms and fuel maps. Injection is accomplished by switching an integrated solenoid valve on and off. Injection delivery quantity is a function of both electrical signal time and plunger velocity. The time or duration the injector solenoid is electrically actuated is one factor determining the injection quantity. This means the longer a signal is applied to the solenoid, the greater the quantity of fuel injected. However, the faster the engine rotates, the greater the distance of plunger travel per unit of time. Consequently, more fuel is injected at higher engine speed if the time the solenoid is electrically actuated remains constant. Above – Fuel is supplied to EUI’s through drilled passageways in the cylinder head known as fuel galleries of rails. The point when the injector solenoid is energized also determines the beginning of injection. Since injection timing is controlled electronically, the beginning and ending is completely variable. 288 289 Overhead cam actuated unit injector and high- mounted cam actuated unit injector. The least amount of injection lag and highest pressurization is produced by overhead cam engines Major EUI Components 290 Suction (Fill) Stroke Fuel passages integrated into the cylinder head supply low-pressure fuel from the supply/transfer system to each injector. Supply circuits are arranged to ensure consistent fuel temperature between injectors. Fuel flows into internal passageways and the pressure chambers within the injector to cool the injector and purge any vapour from the fuel circuits. The injector plunger, driven by the camshaft, moves through a stroke fixed by the injector plunger travel adjustment. On its upward stroke, the low-pressure fuel supplied to the injector fills the cavity created beneath the plunger. Fuel can enter this cavity through the open spill, poppet or needle valve. Main Injection The injector plunger will begin its downward stroke and fuel below the injector plunger will be forced out of the injector through the fuel return passageway. Without a signal from the ECM to the solenoid the plunger will bottom out it stroke and the suction cycle will repeat with no pressurization or delivery of fuel. When the ECM does send an electrical signal to the solenoid, the poppet, spill or needle valve will close the fuel return passageway causing trapped fuel to pressurize beneath the injector plunger. As the plunger continues its downward stroke, fuel pressure builds in the high-pressure passages, the nozzle valve opens and fuel injection begins. Maximum injection pressures occur near the end and not the beginning of the plunger travel, this point usually coincides with the time the solenoid valve is de-energized. 291 Residual Stroke Injection will continue until the solenoid is de-energized. This will allow the spill control valve to open thus, causing the pressure to collapse beneath the plunger. Fuel beneath the plunger vents to the return passage, consequently, the nozzle valve will close abruptly, terminating injection. EUI Adjustment Adjustment of plunger travel is required for EUI’s. The adjustment ensures the plunger travel is almost bottomed out when the camshaft is on its outer base circle or crown. Adjustment procedures vary between manufacturers. Adjustment is performed according to a maintenance schedule. Newer linkage designs connecting the plunger to the rocker lever minimizes wear leading to adjustments that are more frequent. Rather than have a rocker arm slide across the face of a plunger, linkage is designed to pivot with a minimum amount of friction. Above – Adjusting plunger travel is a critical adjustment made to EUI’s. Safety of unit injection Unit injection systems are not likely to be involved in run-away engine operation like mechanically governed systems are. In the event of an injector malfunction, only one 292 uncontrolled injection can happen. For example, if the solenoid valve remains open, no injection can take place since the fuel flows back into the fuel return circuit. Since the valve is open, it is also impossible to cause pressure to build below the plunger. If the solenoid or poppet valve is stuck closed, no fuel can enter the high-pressure chamber. In this instance, only a single injection can take place. Electrical signal processing The EUI with the ECM achieve precise injection timing and fuel quantities using principles of electronic signal processing. Sensors collect data regarding engine position and operating conditions (i.e. intake, fuel, oil, and coolant temperatures, boost pressure engine speed and others), as well, driver demand. The ECM collects and processes the data based on operating algorithms and fuel maps stored in memory to decide what the exact injection timing and fuel quantity are necessary to realize optimal performance, emissions and fuel economy. Pulse width modulation (PWM) There is a variety of output signals produced by the ECM but, the signal operating the EUI’s is especially unique.